The present invention is directed to stretch wrap films and methods for their use. In particular, the present invention is directed to stretch wrap films having excellent maximum stretch, excellent holding force, superior puncture resistance, high total energy dart drop, high cling force and overall strength. The films of the present invention are particularly useful in hand wrapping applications.
The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the unitizing of palleted loads is a significant commercially important application of polymer film, including generically, polyethylene and other polyolefins.
Over-wrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In addition to machine wrapping, these products are utilized in hand wrapping operations. Pallets can be hand wrapped quickly, safely and economically. Hand wrapping is particularly useful in wrapping irregular or non-uniform pallets and articles. Hand wrap films exhibit toughness sufficient to hold the article. The industry also utilizes narrow width hand wrap films. These offer an alternative to adhesive based wrapping films, tape, twine, strapping or glue. The hand wrap films need to exhibit excellent cling force, but only to the film itself and not to the wrapped article. The hand wrap operations offer affordable, easy to use means of keeping loads clean, damage free and safe from pilferage. The hand wrap films should also exhibit excellent puncture and tear resistance.
Hand wrapping large pallets is finding acceptance in industry today. In this method, the film is arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The operator may install the roll of film so used on a hand-held wrapping tool for ease of use.
Some of the properties desired of a good stretch wrap film for hand wrapping are as follows: good cling or cohesion properties, high puncture resistance, good machine direction tear resistance, good transparency, low haze, low stress relaxation with time, high resistance to transverse tear especially when under machine direction tension, producible in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, and low modulus of elasticity.
Currently, different grades of stretch wrap films are commonly marketed for different end uses according to overall film properties. For example, certain stretch wrap films have superior properties for load retention, but these films are characterized by having poor stretching characteristics. On the other hand, certain stretch wrap films having superior stretching properties have low load retention properties, thus limiting their use. Some of the broader categories include: general purpose hand wrap films, heavy duty hand wrap films, and special formulation hand wrap films.
A need exists to develop premium grade hand wrap stretch films. These films are particularly suitable for applications requiring minimum xe2x80x9cneck-downxe2x80x9d and a stiff feel. The products should provide good load retention characteristics and excellent stretching characteristics while still maintaining other important stretch film properties. Such films are used in premium or heavy duty applications.
Accordingly, the present invention provides for multi-layer films for hand wrap stretch applications including stretch wrap films. The preferred resultant film has five layers with each layer being a blend of components to achieve the desired improved results.
In accordance with the present invention, attention has been focused on developing higher performance hand wrap stretch films, while maintaining cost efficient production capabilities. Performance enhancements include improvement of load retention, tear resistance in machine direction (MD) and transverse direction (TD), puncture resistance, ultimate elongation, and overall strength. Desired properties of a xe2x80x9cgoodxe2x80x9d hand wrap stretch film are good cling or cohesion properties, good tear resistance in MD and TD directions, good clarity (low haze), high ultimate elongation, high tensile values, good stiffness (modulus), and high yield per pound.
The present invention is directed to co-extruded films comprised of at least 5-layers construction that are used for hand wrap film applications or premium grade stretch film applications. In one embodiment, these multi-layer stretch wrap films are manufactured as cast films with conventional co-extrusion methods. Each of the five layers comprises a single polyolefin polymer or a mixture of polymers. When a mixture of polymers is utilized in a single layer, the polymers are introduced into an extruder to be mixed and extruded as a single layer. In one embodiment, the multi-layer film comprises a first outer layer, a first inner layer, a core layer, a second inner layer and a second outer layer. Each of the first and second outer layers comprises a blend of linear low density polyethylene, and a metallocenes based linear low density polyethylene. Each of the first and second inner layers comprises a blend of linear low density polyethylene and a low density polyethylene. The core layer comprises linear low density polyethylene products. The core layer preferably comprises a blend of different linear low density polyethylene components. In another embodiment, the first and second outer layers comprise blends of different linear low density polyethylene components. In yet another embodiment, a linear low density polyethylene butene copolymer is utilized in each of the layers.
Another embodiment of the present invention provides a multi-layer film comprising at least a first outer layer, a first inner layer, a core layer, a second inner layer and a second outer layer. The first outer layer represents from 10 to 25 percent by weight of the film and comprises a blend of linear low density polyethylene, and a metallocenes based linear low density polyethylene. A first inner layer represents from 20 to 40 percent by weight of the film and comprises a blend of linear low density polyethylene and a low density polyethylene. A core layer represents from 10 to 30 percent by weight of the film and comprises linear low density polyethylene products. A second inner layer represents from 20 to 40 percent by weight of the film and comprises a blend of linear low density polyethylene and a low density polyethylene. A second outer layer represents from 10 to 25 percent by weight of the film and comprises a blend of linear low density polyethylene, and a metallocenes based linear low density polyethylene.
In another five layer film product of the present invention, each of the outer layers represents from 10 to 25 percent by weight of the film and each comprises a blend of 50 to 80 percent by weight of a first linear low density polyethylene product, 10 to 40 percent by weight of a second linear low density polyethylene product, and 3 to 17 percent by weight of a metallocenes based linear low density polyethylene product. Each of the inner layers represents from 20 to 40 percent by weight of the film and each comprises a blend of 85 to 96 percent by weight of linear low density polyethylene product and from 4 to 15 percent by weight of a low density polyethylene product. A core layer represents from 10 to 30 percent by weight of the film and comprises linear low density polyethylene products. Preferably, a linear low density polyethylene butene copolymer is utilized in each of the layers. In another preferred variation, the core layer comprises a blend of different linear low density polyethylene components. Most preferably, the core layer comprises a blend of at least two linear low density polyethylene products where the first is a hexene copolymer and the second is a butene copolymer. In one embodiment, the first and second inner layers each comprises a blend of a linear low density polyethylene butene copolymer product and a low density polyethylene homopolymer product having a fractional melt index. This blend comprises from 85 to 96 percent by weight of the linear low density polyethylene product and from 4 to 15 percent of the low density polyethylene product.
Another embodiment of the present invention provides a multi-layer film comprising two outer layers, two inner layers and a core layer. A first outer layer represents from 15 to 20 percent by weight of the film and comprises a blend of 60 to 70 percent by weight of a linear low density polyethylene hexene copolymer, 20 to 30 percent by weight of a linear low density polyethylene butene copolymer, and 5 to 15 percent by weight of a metallocenes based linear low density polyethylene product. A first inner layer represents from 30 to 35 percent by weight of the film and comprises a blend of from 85 to 96 percent by weight of a linear low density polyethylene butene copolymer, and from 4 to 15 percent by weight of a low density polyethylene homopolymer product. A core layer represents from 15 to 20 percent by weight of the film comprises a blend of from 20 to 30 percent by weight of a linear low density polyethylene hexene copolymer, and from 70 to 80 percent by weight of a linear low density polyethylene butene copolymer. A second inner layer represents from 30 to 35 percent by weight of the film and comprises a blend of from 85 to 96 percent by weight of a linear low density polyethylene butene copolymer, and from 4 to 15 percent by weight of a low density polyethylene homopolymer product. A second outer layer represents from 15 to 20 percent by weight of the film and comprises a blend of 60 to 70 percent by weight of a linear low density polyethylene hexene copolymer, 20 to 30 percent by weight of a linear low density polyethylene butene copolymer, and 5 to 15 percent by weight of a metallocenes based linear low density polyethylene product. Preferably, the linear low density polyethylene butene copolymer in each layer is the same polymer. Such a polymer has a melt index from 1.7 to 2.3 g/10 min. and a density of from 0.915 to 0.9198 grams per cubic centimeter.
In another embodiment of the present invention, a cling agent is added to one or both of the outer layers. Polybutenes and particularly polyisobutylenes are preferred cling agents. Such agents are added to one or both of the outer layers in amounts from about 0.1 to 2.0 % of the total weight of the final film.